Spinal surgery is a common medical procedure in the United States and throughout the world. Spinal surgery is typically performed to decompress nerves and/or to stabilize the spinal elements. Spinal surgery can involve either anterior or posterior approaches to the spine. There are variations in the spinal anatomy of the cervical, thoracic, and lumbar levels of the spine. However, there are common anatomical elements of the spine at all the levels. The anterior bone elements of the spine include the vertebral bodies, with the interposed intervertebral discs. The posterior bone elements of the spine include the lamina, the spinous processes, the facet joints, the pedicles, and the transverse processes. Several ligaments, including the anterior longitudinal ligament, the posterior longitudinal ligament, the ligamentum flavum (or interlaminar ligament) and the interspinous ligament provide fibrous connections between the bone elements and contribute to the mechanical stability of the spine.
The neural elements, which include the spinal cord, the cauda equina, and the nerve roots, are covered by the dura and is positioned within the spinal canal. The lamina provides a posterior bony roof over the spinal canal and the dura. The lamina protects the neural elements in the spinal canal and also contributes to the mechanical stablility of the spine. The interspinous ligaments and ligamentum flavum (together referred to herein as the posterior ligaments), the spinous processes, and the lamina are important in contributing to the mechanical stability to the spine, especially in forward flexion of the spine.
One of the most common types of spinal surgery involves decompression of the neural elements within the spinal canal. Decompression of the neural elements commonly necessitates removal of all or part of the lamina and the posterior ligaments and is referred to as a laminectomy. Decompression laminectomy involves the removal of the lamina, spinous processes, and posterior ligaments, thereby exposing the dura and nerve roots. After a decompression laminectomy, scar tissue typically forms over the dura and only the skin and posterior spinal muscles protect the neural elements. The neural elements become more vulnerable to traumatic injuries after surgery. Furthermore, laminectomy can result in the compromise of spinal stability, leading to spinal deformity, pain, and neurologic problems including myelopathy or nerve root symptoms.
Following a laminectomy, subsequent spinal operations and revision surgeries are much more difficult than a first surgery on the spine. Scar tissue forming over the dura is a significant problem after surgery and frequently a cause of further spinal pathology symptoms. Because of scar tissue formation, subsequent surgeries are more difficult because of a lack of clear anatomic boundaries between the dura and other soft tissues. Scar tissue commonly forms dense adhesions to the dura. Very careful dissection is therefore required in revision spinal surgery to expose the dura and to separate the dura from the other surrounding tissues. It is not uncommon to injure the dura during these subsequent surgeries, thereby resulting in leakage of spinal fluids and sometimes injury to the neural elements. Various dural coverings and soft tissue grafts have been utilized in the past, however, these coverings are simple sheets that are laid over the dura and that are not anchored to the bone. These coverings can move out of place during wound closure or subsequent removal of deep surgical suction drains. Soft coverings do not provide any significant protection to the dura from penetrating trauma. During subsequent surgical procedures, a simple overlay covering would provide little resistance to pressure from surgical instruments. Also, these sheet-like dural coverings and soft tissue grafts frequently do not result in a clearly defined dissection plane between the dura and the other posterior soft tissues during subsequent surgery.
Decompression laminectomy, particularly in the cervical spine, can result in spinal instability, leading to spinal deformity, pain, and neurologic problems including myelopathy. The lamina and posterior ligaments play an important part in the stability of the cervical spine. It is well known that after cervical laminectomy, a certain percentage of patients will develop a flexion deformity of the spine. Because of this, many surgeons have opted for an anterior approach to decompression of the cervical spine. However, the anterior approaches require a spinal fusion that limits functional mobility of the cervical spine thereafter. Some surgeons, especially in Japan, remove the cervical lamina en bloc and then reattach the cervical lamina back to the spine using sutures or wires in a manner that expands the spinal canal. This procedure is known as laminoplasty. During laminoplasty, the lamina is reattached in a fashion that expands the spinal canal and decompresses the spinal cord. Usually, bone grafts are positioned between the lamina and facets prior to reattachment of the lamina in order to achieve a more expansive spinal canal. Bone growth slowly results in bony reattachment of the removed lamina back to the spine. Several surgeons have reported that laminoplasty helps prevent the development of post-laminectomy cervical spinal deformity. However, the procedure of laminoplasty can be technically challenging, because removal of the lamina en bloc is difficult, and runs the risk that the lamina will move out of place during healing, resulting in compression of the spinal cord.
Another procedure that is performed on the spine is spinal fusion. Spinal fusion can be anterior or posterior. One method of performing a posterior fusion, especially on the cervical spine includes placing a bone graft between the lamina of adjacent spinal levels. The bone graft must be held in place until bony healing and fusion takes place. The bone graft is commonly secured with wires passed under the adjacent lamina. However, this procedure can be dangerous because the spinal canal must be entered when passing the wire underneath the lamina. Pressure on the spinal cord can result in quadriplegia. The method of performing an anterior spinal fusion involves removal of the intervertebral disc and placement of a bone graft. Frequently a plate is attached to the vertebra on either side of the bone graft, to secure that the bone graft does not extrude anteriorly and to prevent motion while the bone graft heals into place.
A recent development in spinal surgery is the implantation of artificial spinal discs. One example of an artificial disc replacement is the Charite artificial disc available from DePuy Spine, Inc. located in Raynham, Mass., USA. Artificial discs may be advantageous in treating disc disease while maintaining spinal motion. A spinal level that is fused is rigid and places increased mechanical stresses on the adjacent spinal disc levels, thereby accelerating the development of degenerative disc disease at the adjacent spinal disc levels. An artificial disc may be more advantageous than a fusion because the artificial disc allows motion at a disc space where it is inserted. Use of a rigid anterior or posterior spinal plate over a segment where an artificial disc is implanted would counteract the benefits of segmental motion provided by the artificial disc. Therefore, rigid posterior spinal instrumentation is usually not used in combination with an artificial disc. Furthermore, implantation of an artificial disc anteriorly between two vertebral bodies usually requires intact posterior spinal elements. Intact posterior spinal elements are important in preventing excessive distraction of the disc space during placement of an artificial disc and dislocation of the artificial disc post-operatively. Patients who have had lumbar laminectomy with removal of posterior elements, for example, the lamina, have therefore not been good candidates for total disc replacement with an artificial disc. It would be advantageous to stabilize the posterior elements while still permitting segmental spinal motion.
An older method of performing a fusion was a posterior midline fusion. This procedure involves placing bone graft on the lamina, adjacent to the midline of the spine. However, complications of extensive bone growth impinging on the neural elements was frequently reported and posterior midline fusion has been largely abandoned. Currently, BMP (Bone Morphogenic Protein) is available for use in the United States and elsewhere in the world. BMP encourages bone growth and the formation of a fusion mass. However, there is no present method or system for placing BMP in a controlled fashion, over exposed dura, after laminectomy. Present orthopedic plating systems are not presently designed to cover over the posterior midline of the spine to protect the dura.
Surgical meshes have been designed for a variety of non-orthopedic uses. Flexible surgical meshes are commonly used, for example, for hernia repair or vascular grafts. There are some meshes or mats that can be attached to bone. One known vertebral auxiliary fixation device has a holding mat and a plurality of fastening elements. Where a foreign object which is implanted in the vertebra, the holding mat is fastened to the vertebra such over the foreign object to prevent the implanted foreign object from jutting out of the vertebra. The holding mat and the fastening elements are made of a material capable of being assimilated into the tissues of a human body. Another known spine stabilization system is designed for application to the anterior spine, and includes a flexible member attachable to a portion of the spinal column. These devices require single points of screw fixation and do not permit sliding.
For the fixation of bone parts or for the bridging of bone defects, particularly in the region of the skull and jaw, it is also known to use a bridge made of biocompatible material with a reticular structure and with recesses for receiving bone screws. Each recess can be used for receiving a bone screw. The bridges do not permit sliding. Orthopedic implants can be made from a mesh material treated in order to promote bone growth, to provide antibiotics, or to provide other beneficial treatment. These devices require fixed anchoring holes for receiving a fastener, and do not permit sliding.
Meshes or mats containing round fixed holes for placement of screws do not give a surgeon flexibility or leeway in screw placement. The distance between adjacent anatomic points, such as a cervical or lumbar facet, will vary from patient to patient, and from one spinal level to another within a single patient. A device with fixed spacing of single round holes may not be capable of being aligned with a patient's anatomy to provide optimal fastening to a patients bone, or simply may not fit a particular patient or spinal level at all, because the patient's anatomy may not line up with the predetermined fixed round holes. Furthermore, fixed round holes do not permit a sliding or gliding motion upon the anchoring screws. Allowing some physiologic motions of the spine may be important in avoiding accelerated degenerative changes of the spine at the levels that are adjacent to a fixation device.
There is a need for a dural covering that will provide a dissection plane between the dura and the soft tissues and help inhibit the attachment of dense scar tissue adhesions to the dura. The dural covering would preferably be secured in place to the bone, so as to not move off of the dura during wound closure or removal of surgical drainage tubes. There is also a need for a posterior spinal implant that simulates the function of the posterior ligaments and lamina, thereby restoring some of the posterior stability to the spine. Still another need is for an anchored covering adapted for use with an interlaminar bone graft, thereby keeping the bone graft in position without having to pass sublaminar wires. There is a need is for an anchored covering over an implanted artificial disc to keep the artificial disc from extruding anteriorly while still permitting spinal motion at the operated disc level. Yet another need is for an anchored dural covering that provides a basic structural framework for the application of posterior bone graft, for example morcelated graft, or BMP (bone morphogenic protein). The structural framework would also preferably prevent bone growth from impinging upon and compressing the neural elements. Yet another recognized need is for a covering that can be slidably anchored to bone, thereby allowing ease of application of the covering and also allowing some physiologic movement of the vertebra. There is a need is for a covering that gives the surgeon leeway and discretion in placement of bone fasteners and that is capable of application to a variety of different patients and different spinal levels. Still another need is for a fenestrated mesh like structure that when covering the laminectomy defect will allow for drainage of blood out of the spinal canal so as to avoid the development of an epidural hematoma. The present invention fulfills these needs and others.